1. Field of the Invention
This invention relates to nonmagnetic steels for use in cryogenic applications. More specifically, this invention pertains to nonmagnetic steels suitable for components in superconducting machinery to which a large static or dynamic load and a high electromagnetic force are applied at cryogenic temperatures below 20 K. Such components as mentioned above are, for example, a rotor of a superconducting rotating machine, a support for a superconducting magnet in a nuclear fusion reactor, and a cable jacket for superconductor in a large scale magnet.
2. Description of the Prior Art
With the development of cryogenic machinery utilizing superconductors, materials of higher performance have been required for use in such machinery. Particularly, the rotor and the magnetic support mentioned above are required to be nonmagnetic and have high strength both at room temperature (about 300 K.) and at cryogenic temperatures below 20 K. Since the scale and capacity of the cryogenic machinery have increased from experimental or prototype one to more practicable one, its main components have tended to be inevitably constructed by welding. Therefore, high strength and nonmagnetic alloys for use in such machinery should also have excellent weldability. Here, "excellent weldability" means that the alloy can be welded without weld defects such as HAZ (heat affected zone) fissuring or fusion zone cracking and the strength of the softened weld metal region can be restored to the level of the base material by post-weld heat-treatment without reheat cracking and reduction in ductility or toughness.
The A286 iron-base superalloy (Fe-26Ni-15Cr-2.2Ti-1.3Mo, Mn.ltoreq.1.5, by weight percent) has been known as a material of this type [see, for example, E. N. C. Dalder, "Development of Forging and Heat-Treating Practices for AMS 5737 for Use at Liquid Helium Temperatures", Adv. in Cryogenic Eng., 28 (1982), pages 883-892].
This alloy meets the aforesaid requirement for the strength, i.e., the 0.2% yield strength of the alloy is 70 to 80 kg/mm.sup.2 at room temperature and 90 to 100 kg/mm.sup.2 at 4 K. which are about twice as high as those of AISI 300 series austenitic alloys. Since the alloy was, however, originally developed as a heat-resistant material, no consideration has been given to its weldability or its magnetic properties at cryogenic temperatures. In other words, the alloy has very poor weldability, and it is difficult to use it in cryogenic applications requiring weldability. In addition, though the austenite phase of the alloy is fully stable against .alpha.'-martensitic transformation even at 4 K., the alloy has the disadvantage that it shows weak ferromagnetism at cryogenic temperatures owing to the magnetic transition of its austenitic matrix. [D. R. Muzyka, "The Metallurgy of Nickel-Iron Alloys", The Super Alloys edited by C. T. Sims and W. C. Hagel, John Wiley and Sons, N.Y. (1972), pages 113-142; J. A. Brooks et al., "Progress Toward a More Weldable A-286", Welding Research Supplement (June, 1974), pages 242-245; and J. A. Brooks, "Effect of Alloy Modifications on HAZ Cracking of A-286 Stainless Steel", Welding Journal, 53 (November 1974), pages 517-s-523-s].
JBK75 iron-base alloy (Fe-30Ni-15Cr-2.2Ti-1.3Mo, Mn.ltoreq.0.1, by weight percent) was developed as an alloy having improved weldability [see, for example, W. A. Logsdon et al., "Cryogenic Fatigue Crack Growth Rate Properties of JBK-75 Base and Autogenous Gas Tungsten Arc Weld Metal", Adv. in Cryogenic Eng., 30 (1984), pages 349-358]. Since, however this alloy is a modified version of the alloy A286 with an extremely lowered Mn level and increased Ni content, the alloy shows stronger ferromagnetic behavior than that of the A286 at cryogenic temperatures below 20 K. The ferromagnetism of these Fe-Ni-Cr-Ti alloys is mainly due to their high Ni concentrations.
Besides the aforesaid alloys of which the main composition is Fe-Ni-Cr-Ti, it may be possible to use existing alloys containing Mn in a high concentration for the aforesaid purpose. If these alloys have such a composition that no .delta.-ferrite is formed in the base material or weld metal, they are advantageous over the aforesaid A286 and JBK15 in regard to the magnetic properties at cryogenic temperatures of the austenitic matrix.
One example of such alloys is a precipitation-hardenable elinver-type alloy (see USSR No. 464658). However, since this alloy has a low Cr concentration and a high Al concentration of about 1%, the hardening rate during aging is very high and the optimum aging time is several hours at 700.degree. to 750.degree. C. It is not suitable, therefore, for use for a superconducting cable jacket or large scale structural components. Moreover, the age hardening characteristics of the alloy and lack in the consideration for trace elements such as C, P, S, Si, and B result in very poor weldability, i.e., the alloy is highly susceptible to hot-crackng by welding and reheat cracking upon post-weld heat-treatment.
Another example is a series of heat-resistant alloys of which main composition is Fe-Ni-Cr-Mn-Ti (see U.S. Pat. No. 3,201,233). As will be shown below, if the concentrations of the respective trace elements is tightly controlled as in the case of this invention, some of these alloys may have a possibility to show similar magnetic and mechanical properties at cryogenic temperatures to those of the alloy of this invention. The ductility at cryogenic temperatures of the alloys, however, is lower than that of the alloy of this invention and tends to decrease with an increase in their Mn concentration. Furthermore, these alloys has the disadvantage that the cellular precipitation of .eta.-Ni.sub.3 Ti occurs in the weld metal region of their weldament by post-weld heat-treatment comprising solutionizing and aging. This precipitation markedly reduces the ductility and toughness of the weldament at cryogenic temperatures.